The present invention relates generally to microelectronic circuit packaging apparatus, and, more particularly, to a warp-resistant heat sink suitable for soldering to a relatively fragile, ceramic semiconductor substrate.
In the microelectronic circuit packaging technology, there is a significant trend today towards the use of multilayer printed wiring boards, particularly of the ceramic type. Tens and hundreds of semiconductor chips may be mounted on one surface of such a substrate measuring, for example, several inches on a side. These microcircuits consume a relatively large amount of power, and accordingly generate considerable heat. As the packing densities of heat-generating microelectronic circuits become increasingly higher in order to achieve savings in component costs and increases in operating frequencies, there is greater need for effective heat dissipating mechanisms to ensure the operation of such circuits within prescribed limits. In order to prevent excessive heat build-up, and resultant damage to the microelectronic circuitry and packaging apparatus, it is necessary to artificially cool the substrate boards.
Heat sinks are familiar devices in the circuit packaging arts for dissipating excessive heat generated by operating electronic components to the ambient environment. It is known in the prior art to utilize metal fins soldered or otherwise fastened to electronic components or packaging boards to dissipate surplus heat. The ceramic substrates in current use in microelectronic circuit packaging technology, however, present particular problems regarding the design of compatible, efficient heat sinks. Such substrates are relatively fragile and will crack or break if subjected to undue mechanical or thermal stresses. Such substrates must also be capable of dissipating heat from substantially the entire surface area of one side thereof, since the opposite side of such substrates is ordinarily totally occupied by heat-generating microelectronic circuitry. The fact that the interior of such substrates comprises one or more networks of signal and power lines, which also generate heat, is another reason compelling the use of a large heat sink occupying substantially all of one side of the substrate.
While various metals have been used in heat sink fabrication, copper has been found to be an excellent choice, because of its heat conducting properties. Copper, however, has a thermal coefficient of expansion which is substantially different from that of the material used in the construction of ceramic substrates. For this reason, heat-dissipating copper fins of any appreciable size cannot safely be soldered or otherwise fixed to the ceramic substrate, since the heat warpage resulting from the dissimilar thermal ratios of the copper and ceramic material eventually causes sufficient mechanical stress on the ceramic material to crack or break it, resulting in damage to the conductive lines therein and failure of the circuit package.
During the fabrication step of soldering relatively large copper fins to a fragile ceramic substrate cracking or breaking is also likely to occur, because of the dissimilar rates of thermal contraction of copper and the ceramic material experienced as the package cools from the soldering temperature. Even if breakage does not occur as a result of excessive heating or cooling, the fins soldered to the substrate have a tendency to creep out of place under the influence of mechanical stress and to eventually become dislocated from the substrate, resulting in the need to remove and repair the microelectronic package.
Therefore, it is an object of the present invention to provide a warp-resistant heat sink suitable for mounting on a relatively fragile, ceramic substrate.
It is also an object of the present invention to provide a warp-resistant heat sink of approximately the same dimensions as the substrate to which it is mounted.
These and other objects of the present invention are achieved, according to one embodiment thereof, by providing a rectangular plate formed of a metal having a thermal coefficient of expansion substantially identical to that of the substrate material. Slit-like openings along opposite edges of the plate receive U-shaped, copper, heat-conducting members, which members are all soldered to the same side of the plate. The plate, in turn, is soldered to one surface of the substrate. The resulting laminate comprises two outer layers of material having substantially identical thermal coefficients of expansion and an inner layer of copper having a different thermal coefficient of expansion. As a consequence, the laminate has relatively symmetrical thermal expansion and contraction properties, and the warpage which would otherwise result from the bonding of two materials having dissimilar thermal coefficients of expansion is substantially reduced if not totally eliminated.